I am building a world in which technology has taken an enormous step back due to an apocalyptic event. For one reason or another guns no longer exist in the world and I would like to have a reason why they could not be created/used again, even given enough time for technology's advancement once again.

I would like to leave the atmosphere alone but maybe remove a resource that would in-turn make the production of guns impossible. I wouldn't mind if ammo was the cause of guns not being used instead.

I don't know much about firearms so I'm not sure if what I have explained makes much sense but any help is greatly appreciated! Cheers

$\begingroup$This idea keeps popping up here. It's cool, and that's great, but everybody playing with this idea needs to know what nitrocellulose is, how it works, and that it can be made so easily that some people come across it by accident.$\endgroup$
– Sean BoddyJul 31 '17 at 1:21

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$\begingroup$I think you need to delineate what is a gun in your world and what is not. A gun can be described in very broad terms as a single piston internal combustion engine. Anything that can produce a flame (deflagration) and another thing that can contain that flame (metal, or even wood), can be used to project an object.$\endgroup$
– WolfgangJul 31 '17 at 2:00

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$\begingroup$There is no way to make guns impossible with mind-bending changes to physics. A more practical approach would be that as soon as someone makes a gun, a demon appears, and eats them. Feel free to replace "demon" with KillBot or whatever.$\endgroup$
– Tony EnnisJul 31 '17 at 23:13

31 Answers
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The Japanese Samurai were enthusiastic proponents of firearms. One of the most dramatic scenes in a Japanese movie is the ending of Kagemusha, which recreates the Battle of Nagashino, where the Takeda clan was essentially destroyed as a power by the effects of volley fire by Oda Nobunaga's troops.

Volley fire isn't fun if you are running into it

So if firearms were so effective, why did they essentially disappear from the Japanese arsenal during the Tokugawa shogunate? The reason is cultural.

Samurai, like European knights or Ottoman Janissaries were highly skilled warriors who started their military training in boyhood and were not required (indeed forbidden) to engage in trade or crafts. With a lifetime of training and the ability to practice their arts on a daily basis, they were fearsome warriors who could defeat any opponents who were not skilled or equipped the same way they were. Peasant armies were little more than rabbles or (at best) spear carriers who could form a line to slow down opponents as you got organized or regrouped.

Firearms were the key to the "Infantry Revolution" which started in the mid to late 1400's, as weapons and tactics were developed which let large numbers of unskilled men take to the field and effectively fight against Samurai, Knights, Janissaries or others who's prowess depended on long training and practice. Firearms swept away Knights and Janissaries in Europe and the Middle East, but Japan, being much more isolated and insular, was able to block the importation of firearms and forbid the local manufacture, allowing the Samurai class to retain its power and not fear being overthrown by a peasant revolt.

So in your setting, there must be some cultural reason to fear or forbid the manufacture or development of firearms. If the apocalyptic event took place and required large numbers of people to shelter in makeshift structures for protection from the environment, then there might be a well founded cultural fear of firearms, since using firearms in a flimsy shelter could cause a breach and allow deadly radiation or whatever other environmental effects caused the apocalypse to enter. After several generations, any pre existing firearms would probably have deteriorated to the point they don't work anyway, ammunition would have decayed and no one in their right mind would even consider recreating them.

So the point isn't to create some sort of mind bending alteration of the laws of physics (preventing the chemical reactions that cause explosions probably means invalidating the chemical reactions needed for life to exist), but to explain why people don't make firearms anymore.

$\begingroup$Except that Japan never lost the ability to make firearms, and kept the knowledge of how to use them. It is true that firearms were not used much during the Tokugawa period, but then no weapons of any kind were used much during the Tokugawa period, when Japan exemplified that most rare kind of human society, a militaristic country which did not make war, foreign or domestic.$\endgroup$
– AlexPJul 31 '17 at 10:48

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$\begingroup$Also, as generations progressed, stories of the dangers of firearms would progress and exaggerate from simple warnings to grandiose fables and legends where whole communities were wiped out because of a single errant shot, and anyone who even brought voice to the thought of using a firearm would be considered insane and dangerous, and ostracized by society.$\endgroup$
– Doktor JJul 31 '17 at 13:36

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$\begingroup$@DoktorJ Isn't it usually the other way 'round? If nobody ever sees these catastrophic consequences of X, warnings about X will soon appear hollow ancient fairy tales.$\endgroup$
– xLeitixJul 31 '17 at 13:58

$\begingroup$For an example of the renunciation of technology in response to the horrors of war, see the Simplification after the Flame Deluge in Walter Miller's A Canticle for Leibowitz.$\endgroup$
– MissMonicaEJul 31 '17 at 16:07

$\begingroup$Actually by the end of the Sengoku Jidai, the Japanese light infantry corps, the Ashigaru were well drilled, equipped and effective. They were the backbone of the Japanese invasion of Korea, where the Koreans discovered that while they had good archers, bows were simply useless against guns.$\endgroup$
– Haakon LøtveitAug 1 '17 at 8:14

The problem is that a shotgun is actually very easily to build, very poor people in the brazilian slums have build them from pipes. Take a look at break action shotguns, you need essentially only a pipe, shotgun munition (a primer with propellant and lead/steel balls), a breech face (a surface on which the munition can explode) and a trigger.

Rifles are much harder to produce because you need rifling, a helical groove which must be cut by tools into the steel. I daresay that a sniper rifle needs such a precision that it cannot be done.

Some proposals:

The apocalyptic event, a war, happened after a time when most of the easily extractable natural resources were declining. The war was making extensive use of neutron bombs which have irradiated cities and most places which contained metals, making them unusable through neutron activation (The radiation may now have ceased, but people retained an irrational fear of everything metallic).

The formula for the propellant got lost / there are no known natural resources for it. The thing is, you cannot use explosives as propellant, it will only destroy the gun. What you need is either gunpowder, guncotton or cordite which have low brisance and are more pushing than destructive.

Humanity dumbed down and/or overspecialized themselves. If computers/roboters are producing better quality with less prices and no payment, humanity in future may use their day to f...play board games with each other and party. Who knows currently how a television functions and even if you know it, could you build one? Or future humans are much too overspecialized: You need an alloy expert, a drill head expert etc. etc.

$\begingroup$I would deem it pretty unlikely that there are no resources to produce gunpowder, as the components can be extracted from manure (Saltpeter), or synthesized as long as you have wood (charcoal). Sulfur is easy to find, and the only component where there could be problems.$\endgroup$
– Doomed MindJul 31 '17 at 7:48

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$\begingroup$Note that those pipes you mention are already the result of high-quality metallurgy - you couldn't make a shotgun with a pipe from, say, the time of Ancient Rome. Rifling was made by hand long before machining - the tricky part isn't the rifling, it's the ammo; it doesn't work with the balls used in early weapons, and other kinds of ammo are hard to produce by hand. Sniper rifles also existed long before industrialisation - including the very effective air rifles; they were rare, not non-existent. Even in WW2, sniper rifles were just the best "normal" rifles, taken off the same line.$\endgroup$
– LuaanJul 31 '17 at 10:20

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$\begingroup$@Luaan You could make a gun with an Ancient Roman water pipe. You just couldn't put nearly as heavy a load in it, mostly because they were made of lead. Also, rifles work just fine with lead ball ammunition, however, prior to the invention of the minnie ball it took a fair amount of effort to cram the bullet through the riflings during loading, making them slower to shoot than muskets. Of course, there are tricks one can use to get around this when speed is more important than accuracy.$\endgroup$
– PerkinsJul 31 '17 at 17:34

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$\begingroup$MythBusters proved that a usable canon could be made from a hollowed log with minimal reinforcing$\endgroup$
– pojo-guyAug 1 '17 at 7:58

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$\begingroup$> The thing is, you cannot use explosives as propellant, it will only destroy the gun. This is completely false. Combustion generates pressure, and by directing pressure you can propel a bullet. If the pressure has no where to go, it explodes. The same black powder can power a gun or an explosion.$\endgroup$
– RyreAug 2 '17 at 5:21

In order to make guns impossible, you need to not only remove the capability for any matter to explode (Which has some pretty serious impacts elsewhere), but you also need to remove the capability for doing things like compressing air. There's also things like Coilguns and Railguns, which use magnetism as the propulsive method.

Honestly, changing the laws of physics to limit the speed an object can go before "Bad Things" happen would be better than making explosives, electricity, compressed air, and however many other things that can propel a bullet impossible.

There won't be a good explanation for guns not working, so simply saying "The laws of physics changed and now guns don't work" is probably the best. In fact, this could be part of the apocalypse in itself. S.M. Stirling's Emberverse uses exactly this scenario, starting from Dies the Fire electricity and fast chemical reactions like explosives and propellants don't work. No electricity, no guns, no engines, everyone ends up using medieval technology.

$\begingroup$Similar thing with electricity in "Fitspatrick's War". A host of satellites were set up to disrupt any electrical systems to the point of unstable overload, making them effectively useless. A pretty terrible narrative handwave, but certainly made the way their society avoided using it interesting. Idiot savants for calculation, clockwork and steam for utilities, and super-advanced chemistry and materials science for blowing things up.$\endgroup$
– tzrlkJul 31 '17 at 1:55

$\begingroup$@Tzrlk read up on EMP, while it wouldn't create a permanent lack of electricity, a large scale EMP event would render pretty much any electrical system not hardened to resist it (which is only done with some military and aviation grade systems) useless. It can be repaired, but current estimates for even an EMP event that just disables the US electric grid are for years to restore it, a worldwide event would take decades to repair and by that time modern civilisation would have collapsed to the point where the knowledge on how to repair it would be gone.$\endgroup$
– jwentingJul 31 '17 at 10:26

$\begingroup$This sounds a lot like one of the linked answers... :) +1$\endgroup$
– MazuraJul 31 '17 at 14:34

Perhaps because guns make a loud noise? But this completely depends on your world.
For example, in a zombie-apocalypse world, it is not specifically good to use loud guns.
Or when a superior force is monitoring the world, for example, the apocalypse was initiated by beings from space, and even though they presume nothing survived, apparently some did. But somehow, using guns will attract their attention.

But the disadvantage of this is that explosives should not be used either.

Maybe there are just way more effective ways to kill someone with the given resources.

Another reason can be that people don't dare to kill each other, because there are not many left, and therefore the human race itself is in grave danger. But I'm not sure where you're heading with your world without more context.

By exaggerating the sensitivity and power of the reaction, you could make gunpowder too dangerous to be practical. Only alchemical specialists and death-seekers would ever chance slapping that stuff together and lighting it up in their hand.

There are a couple ways to accomplish this.

1) Scale down every one and every thing: By jacking the square-cube rule, you can make everything in the world seem physically stronger and more intense.

2) Too much static charge: Whether it be home-grown or cosmically sourced, high radiation can produce excessive amounts of static charge on anything dry, which causes disasters for anyone making or using many forms of gunpowder.

3) More reactive environment: As easy as raising the oxygen content in the atmosphere. Perhaps some atmospheric nitrogen has been converted to nitrous oxide, which increases combustion reactions while not being flammable itself.

$\begingroup$A more reactive atmosphere wouldn't affect the usability of gunpowder or other explosives as they contain their own oxidizer that is already thousands of times more concentrated than atmospheric oxygen. The most it would do is make explosives more likely to start fires.$\endgroup$
– PerkinsJul 31 '17 at 17:43

$\begingroup$I think this one is the best, but probably is better to not provide an explanation to it. Just, 'physics changed'.$\endgroup$
– dmcontadorAug 1 '17 at 7:06

In a post apocalypse that shattered society but didn't destroy the refined metals, one could build a firearm from the scrap metal of destroyed buildings and vehicles. Black powder is easy enough to make: charcoal, sulfur, and ammonium nitrate that can be found in animal manure.

Even in an apocalypse that destroyed all technology and refined metals, just the knowledge that a firearm can be built from raw materials insures that they will be built.

The materials to build firearms existed in pre-firearm times. What they didn't have was the knowledge that such a device was possible, and how to build it.

So, your apocalyptic event would have to wipe out not only the technology, but the accumulated knowledge of the possibility and construction of a firearm.

Instead of making guns 'un-creatable', why not make the use of projectile weapons in general be too inaccurate to bother with for any real purpose?

The way I'd propose that, is that your hypothetical apocalypse has altered the weather patterns so severely, that there is a constant gale-force wind blowing, which changes it's direction frequently and without notice.

In a world with constant high wind, no rifleman etc would be able to land an accurate shot, and pretty soon people would seek alternative methods to kill each other. Sure, you'd get the occasional defensive position with a machine-gun perhaps, where accuracy doesn't matter. But in a hunting situation or an assault situation where accuracy is important, even the heaviest of projectiles would be severely deflected in say a constant 80-100kph gale.

$\begingroup$This would be a problem if the OP wants other projectile weapons like bows or crossbows.$\endgroup$
– Doomed MindJul 31 '17 at 7:57

$\begingroup$@DoomedMind agree, this would only really be suitable if OP wants to limit their world to melee warfare. Of course the other issue I've thought of with my example, is that in the situation where there's a constant gale on the surface, people would likely retreat below the surface- and then things like shotguns would have value again. So it's not a perfect option really.$\endgroup$
– Aaron LaversJul 31 '17 at 8:06

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$\begingroup$For an averageish .308" projectile from a rifle, 62mph (100kph) winds would result in <5 inches of windage at 100 yards, and ~0.3 inches at 25 yards. That assumes the winds are perpendicular to the line of fire; the effect becomes mixed with the drop at other angles. Regardless, at short range, the winds just don't have enough time to act on the projectile to matter.$\endgroup$
– Jay KominekJul 31 '17 at 19:25

$\begingroup$Yeah, I've always grown up swinging a .22 or .223 to chase foxes with, so wind was always problematic when shooting over long distances. Wasn't an ideal solution, but maybe a limiting factor for OP's story :)$\endgroup$
– Aaron LaversAug 1 '17 at 1:35

A post apocalyptic society may be an anarchic one without any sort of major population centres or organisations - and so large organised industry as a whole might no longer exist.

So now, while you can create a firearm using the Brazilian pipe method from Thorsten's answer, I imagine these guns are not too accurate at any appreciable range, and slow to reload - compared to say, a well-crafted bow in the hands of a skilled archer. Remember, early guns were basically just good enough to blast a large shower of metal balls at a row of men and hope to hit as many as possible to reduce the size of the shower of metal balls they were going to blast back at your row of men - an individual shooting at another at a decent range was a more difficult matter. But in the post-apocalyptic scenario, large groups of people simply don't war with other large groups of people.

Now, with farms, supermarkets and fast food restaurants all wiped out by the apocalypse, we've gone back to the age of hunting (assuming a lot more wild animals survived than humans - maybe in the jungles previously uninhabited by humans which were therefore not targeted by the apocalypse) and we've all become good at archery, and blade type combat, making home-made guns a lot less effective.

$\begingroup$If you go by the Japanese invasion of Korea, you see what happens when guns and spears go up against bows. Bows lose. Really hard. If you look to Europe, you can see how immensely useful guns are in sieges, and how cannons will be an improvement on siege weapons. And from there you will eventually get a hand cannon and from there, an arquebus. Guns are just really, really deadly. And the second there are enough humans together to make a wall, there will be other humans wanting to have the ability to remove said wall.$\endgroup$
– Haakon LøtveitAug 1 '17 at 8:16

$\begingroup$@HaakonLøtveit - In large organised battles, yes. But in a post-apocalyptic scenario societies might be a lot less organised with people spread few and far between, maybe some will gather together in tribes, but overall, without large-scale industry or large armies. Then it's an individual with a primitive gun they made out of a pipe, some gun-powder and a lump of metal - they might get off one inaccurate difficult shot before their enemy closes on them with a knife or looses an arrow at them, and even then only if they are lucky enough to have the element of surprise.$\endgroup$
– colmdeAug 1 '17 at 8:25

$\begingroup$Having said that, my scenario may be impossible given the caveat to the question, "even given enough time for technology's advancement once again" I suppose assumes the reorganisation of society into one that can form large-scale industry.$\endgroup$
– colmdeAug 1 '17 at 8:32

$\begingroup$If you wish to go by tribal warfare, it's worth noting that native american/indian tribes adopted the gun as soon as they could. It was a lot better for their needs than bows were. For a lot of the same reasons that eurasian steppe peoples adopted firearms as soon as they could too.$\endgroup$
– Haakon LøtveitAug 1 '17 at 9:27

$\begingroup$@HaakonLøtveit - but did they make them themselves, or did they capture/buy them from more technologically advanced cultures? And when did they adopt them? Was it when they were proper, reasonably accurate rifles, or when they were arquebus-type pipes, powder and metal balls?$\endgroup$
– colmdeAug 1 '17 at 13:39

If everyone used energy weapons that were abundant and had clear advantages over guns, then guns could have been quickly phased out. Example: Lasers that can be pass through various types of mater and take out someone makes amour and cover irrelevant.

If those weapons become dominant for long enough, they total eclipse combustion firearms. Guns go the way of the crossbow. A novelty that has no business on a "modern" battle field. With that the collective populous forgets the details of how the produce firearms. I assume laser making requires far more advanced infrastructure the the apocalypse can provide.

$\begingroup$Except that there are 150+ year old firearms that are still being shot today. And 125+ year old firearm designs are still produced today. And "ancient" weapons like crossbows were used in WW2 and Viet Nam - masterkan.wordpress.com/2015/01/31/…$\endgroup$
– ivanivanAug 1 '17 at 20:07

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$\begingroup$I have no doubt that some guns would still exist, but the idea is that the specifics of how they work is forgotten by enough people that they can not be feasibly reproduced.$\endgroup$
– OverthinksAug 4 '17 at 8:54

$\begingroup$@ivanivan - I might, maybe, be able to make a crossbow, but I went looking, and I think few people could say the same outside of artisan craft-folk. Same for other weapons like atlatl, blow-dart, or sling (using more than crafting for some). Even if they were still made and used - as all the above weapons are - if they aren't in general use, few people would know even the basics and fewer could get to usable weapons. With losses from apocalypse, guns could be remade if closer to bow (known in hobby) but if they end up more like atlatl (largely forgotten), they could also be not.$\endgroup$
– MeghaSep 22 '17 at 22:26

You're obviously not a StarTrek fan. In the original series, Kirk mixes some sulfur, coal, and (I think) saltpeter and makes black powder on a world created by god-like beings to test his intelligence (or some such thing). As already answered, you will have to change the Laws of Physics to eliminate guns. In the movie No Country For Old Men, the "bad guy" uses compressed air (and projectiles) to kill people, so you can't just eliminate gunpowder and have your people gun free. At close range, cross-bows are as accurate and as penetrating as small arms. And then there's sling shots and atlatl... I'd say you're out of luck; rapid motion can result from too many processes to be eliminated in anything similar to our Universe.

$\begingroup$@Mathmagician not sure where you get that from. How do you think people produced black powder in the 14th century?$\endgroup$
– Doomed MindJul 31 '17 at 7:55

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$\begingroup$@Mathmagician how much devastating power is sufficient? And how can that not be achieved by simply using more crude blackpowder?$\endgroup$
– BurkiJul 31 '17 at 10:23

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$\begingroup$@pipe No, that does not seem to be the reason. As far as I can see, the reasons were more in the line of limited production capabilities, restructuring of military thinking (as gunpowder changes the battlefield in almost every way), logistics and the weakness of gunpowder to humidity. The weapons themselves weren't very accurate either, both due to the missing rifling and the imperfections in both powder and ball and only got much more efficient when they "invented" volley fire.$\endgroup$
– Doomed MindJul 31 '17 at 10:54

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$\begingroup$Where did Mythbusters find a Gorn to test it on?$\endgroup$
– workerjoeAug 1 '17 at 15:35

Instead of removing explosives (there truly are too many ways to make explosive pressure), what about removing the manufacturing materials?

Virulent bacteria that attack refined metals. If iron and steel cannot be relied on for their strength, making safe firearms would be very difficult. There are materials that could possibly be used, but none with the strength to weight ratio of steel. Brass cannons would still be possible, unless the plague was extended to them as well.

Of course, this has pretty extreme ramifications. Anything made of steel would collapse, this means bridges, buildings and other important infrastructure. Even wooden structures using iron nails would fall apart, assuming the bacteria can get to them.

Blades would need to be made of stone, glass or ceramic. Armor would be wood, leather and bone. If humanity retained the ability to make plastics, or to recycle existing plastic, then armor and gear could do well made from that as well.

I think that the only real way to get a whole world without guns is to regress to the point where you don't have the means to really make them. Remember that basic gunpowder is not easy to make (the process is complicated), but that the ingredients are not hard to get hold of.

So I can see a few possible contenders for the NOGUNS scenario:

Guns are possible, but they're future-tech

This implies that yes, we can make guns, in theory. However, we don't have the practical means yet. We may not have the population centers to get the division of labour required to kickstart the gun trade, the metallurgy may not be there just yet (and what little good steel you have been able to make is needed rightnow for other needs, like say, maintaining a large dam that's essential for your agriculture, keeping a bridge up or what have you. This is in my opinion an interesting opportunity because guns are now a possibility, your intrepid rag-tag band of heroes may at some point come into contact with guns, etc.

Guns are possible, but they're unaffordable

This means that there are people with guns, but these people are few and far between, guns are really expensive, and anyone making them without license are clamped down upon. Imagine The King's Guard having rifles, and nobody else. A point to remember here though is that while muskets are not that hard to make, proper modern rifles are impossible without precise machining and measurements. This will give you a setting where there's a reason why there aren't any guns around. Do you want to fight the King's Dragoons riding out to slaughter your entire village because someone went bird-hunting with a shotgun?

Guns are possible, but people don't know how.

Basically, yeah, you come across Hjemmeladeren, an immense bible about loading your own guns. It tells you exactly how to load, what pressures to use for each bullet, etc. If it's about putting gunpowder into a holster and using said powder to propel a small bit of metal into people/animals/others, the book has the best recipe for the purpose. Only problem is, it's in Norwegian, and your society don't read that language. Of course, if you could, you'd know that Raufoss Ammunisjonsfabrikk refers to what's today called Nammo A/S and in your post apocalyptic world would be a smouldering ruin. (Actually, the smouldering is optional.) So even the knowledge you do find is of little use. This would be a temporary solution, so you should set it to happen relatively early after the apocalypse has passed. This also assumes that the sort of people who would know how to make gunpowder from scratch is gone, or if they're still around, are not talking to the heroes. They may in fact be very insular people living in fortified compounds and have little to do with the outside world. (This does give you a Deus Ex Machina if you need it. Chekhov's Prepper Compound?)

Guns are impossible because resource lack

This means that there's none out of one of these:

Charcoal

Sulfur

Salpeter

Because that's how you make blackpowder. It's simply ridiculous. People have also made cannons out of wood and stone. If everyone lives on a boat, then yeah, I can see this being a theme. But if they're on mainland? Can't really see it.

Guns are impossible because laws of physics

I really don't think that this is a good solution. Basically there will be a bunch of unforeseen consequences from this. A lot of unforeseen consequences. If everyone is bigger and stronger? Why do they have horses? How do they get enough food? Etc. I think it's kind of a cop-out.

Guns are impossible, but nobody knows why

At least here you're kinda being honest about it. There's a reason and you totally have that reason planned out, promise bro. But you're not going to tell anyone because it's a plot point. It informs how the story is told, so telling people gives away some of the story. This is a common one to use if you're GM'ing. Just tell them straight up before the game starts that stuff like guns don't work, and that there's a reason for it, but your characters would have no way of knowing that reason. You can admit OOC that it's because it messes with the story you want to tell, but that's fair. Now everyone's on the level.

But for like a written story? It's kind of unfair, I feel. But I'm not the boss of you know, I'm not the boss of you know and I'm not so big~~~

Personally, I'd have to ask myself why I wanted no guns. If I wanted to tell a story with no guns, but would be fine with guns eventually (after the story was conveniently over) reappearing, then any variation of guns being possible but aren't there yet will do the job, and give you more possibilities.

There is however, another question. WHY do people want guns?

Well, guns are really, reallyreally useful. Big guns, such as cannons can hit fortifications further away than catapults etc. can. They're easier to transport. They are easy on your logistics. (You need to bring gunpowder, you don't need a separate team of skilled carpenters to make sure the catapults work.)

If your enemy clumps up, you can smash a lot of them with cannon-fire. If your enemy disperses, you can clump up and smash them. If you're at sea, cannons are simply better than anything else until we get to the modern era. Arquebuses have a longer effective range than bows (as amply demonstrated by the Imjin War), and they're much more deadly, and are harder to armour against. And as armies start to get larger and thus you need to shoot more people, it's important to note the logistical aspects of bows: You need a well trained man, who's trained a lot with a bow, to get an at-all effective archer. You then need to get him arrows. Arrows are large, expensive creatures.
Look at them!
That's a huge piece of wood, and you need to get a good craftsman to make a whole bunch of these suckers.

Meanwhile, an early bullet is just a small lead ball. You also have some gunpowder. Both of these are cheap (lead is inexpensive, steel warheads on arrows are not. Then there's the wood, feather and craftsmanship in addition...), and the package is much smaller than an arrow is per shot. yes, you need wicks for your gunners, if you use that type of firearm. However, bowmen need to have strings for their bows, and crossbowmen need a bunch of tools for their weapon too. It's not a close competition. In addition to that, you don't need as much raw strength to wield a firearm. So your pool of potential recruit is much larger. You can use the big lads to smack your enemies with big sticks instead.

While yes, a well drilled gunner will be much more effective than a relatively fresh recruit, a fresh recruit with a gun will be more effective than a fresh recruit with a bow, all things being equal. (Which they often were not, since a lot of peasant hunted with bows.)

Finally imagine being besieged. You won't be able to get more supplies, so you think yourself lucky that you have gunpowder based weapons, since you can have more ammunition that way. If the enemy brings archers, they cannot engage your gunners, because your gunners can easily stand ready and fire on a moments notice, which the bowmen cannot. In a siege, firearms are ridiculously overpowered.

In other words, as soon as someone manages to figure out how to gun, there will be guns.

In its most basic form, a gun is nothing more than a pipe bomb with one loose end cap. All the explosive force gets directed one way so it's controllable. There were certain early gun designs that were pretty much exactly this. Think "Roman Candle" only optimized for mounting on a stick and hurling a shower of red-hot metal fragments.

Knocking people's knowledge down far enough that they wouldn't know how to build one would be pretty easy. Even among the people who know the formula for black powder off the top of their heads, how many of them actually know where to find the ingredients and how to refine them into something pure enough to not have all kinds of reliability problems? And figuring out something strong enough to contain an explosion but still light enough to be mobile would also require a lot of experimentation.

Getting it to the point where someone with enough time to experiment can't develop one though... You basically have to do something drastic. Change the laws of physics in some subtle way, and manage to do it without causing everyone to die... It's not easy... But here's a few ideas that mostly focus on "won't":

Keep the tech level high. Society's a wreck, the world is a mass of warlords fighting for supremacy, but just prior to the apocalypse humanity discovered how to make personal shields, and they've managed to scrape together enough infrastructure to continue building them. They reliably stop small to medium objects travelling at more than 20mph. Bullets are no longer effective weapons except for hunting animals. Warfare consists of shielded and armored warriors hitting each other with big, heavy objects.

Make the atmosphere full of explosive gas pockets. This won't prevent all use of guns as some types won't be hot enough to ignite it, but it will make using any kind of fire dangerous and expensive which will keep your society quite primitive.

The aliens saw the flash from all the nukes and are going to make sure humanity does not take their aggressive nature to the stars. They don't interact with us in any significant way except to watch us from orbit and throw lightning bolts at anyone with weapons more advanced than the Roman era.

The apocalypse wasn't a war, it was the invention of a technology that made metal practically useless. Some small, portable, cheap machine that could turn metal into the consistency of clay and back again. Metal became easy to work, but could no longer be used for critical tasks because crazy people could just puttify all metal within a mile radius. The ancient roadways are still lined with the slowly-sagging remains of vehicles, abandoned by their owners and destined to ooze themselves into puddles over the next several centuries. The metal softening machines are long-since lost, but there's still a strong cultural taboo against using metal for anything other than decoration. Without metal, guns are possible, but very, very dangerous.

Edit for more clarity:
I think that there are various options but they are not all so equal when it comes to basic human behavior and needs, or setting consistency:

Actually, if I was a guy meant to mess things up, legends advertising the dangers of firearms wouldn't deter me using these in any way, all the opposite way actually. So culture is out. Remember some people just want the world to burn, even if they have to immolate to start this.

Bacteria or tech eating metal won't prevent guns being made from ceramics or stone pipes? (I read some early cannon barrel where made in stone and as some pointed out, there was bamboo)
Chemistry change would be prejudiciable against life.
An alternative here would be something acting against explosives, but that would be quite strange to have some many varieties of compounds to be targeted. Plus, you have to take into account "explosive like" compounds. They are not explosive per se; not displaying enormous heat and flammes and so on, but they can generate high pressures modifications. And that may be done by disrupting various compounds to generate N2 for example:
http://sciencing.com/can-create-nitrogen-gas-8121054.htmlhttps://www.enotes.com/homework-help/chemical-reaction-between-n2h4-h2o2-produces-595660
(granted, reactions here are exothermic, but not to the same extent)
Also, we have to expect people to find other means. Explosive are not the only ones to help pushing bullets. You could expect having remnants of army armed with railguns soon after if that would help them wiping the enemy forces. We also have to know if you also want siege/vehicle gun weapons to be impossible.

It leaves physics or magic, or shelter settings as some answered (which is quite sensitive answer), with maybe the twist it could be enforced (think Computer Friend in Paranoia RPG;)).

Physics could be changes in laws of motions (or their parameters; for some reason, gravity change that makes you sensitive to reactions of guns. Reason you won't use it into space for example). That won't make guns impossible but impractical.

Magic could be related to the apocalyptical event. I read a book where the world of faeries had stopped living with the invention of powder, but then it was back with a vengeance...
In the "not impossible but impractical", we could imagine that the "common opponent" would not be sensitive to guns, or that it would be dangerous to use guns on them (spreading diseases by bullets wounds, for example, still in this case, forget chopping and smashing too. Don't do like Pacific Rim, where they say using weapons frees dangerous compounds, then proceed by mincing them with giant chainsaws...)
Or that there would be some esoteric reason for which gun use would put some curse over you; and being cursed would be something bad enough for you not wanting to tempt the devil (not like "you will go to Hell when you dire"; which rings like "yeah, sure", to players, because they don't care about what happen to their characters once they are dead, they care about what happen to them when they are playing). Like hordes of demons wanting to eat or mutilate them and make them miserable -hence ironically participating to peace in the world-).

There could also have technological reasons: making them mostly useless.
Like outdated by other weapons, or something like "all people are immune because of some tech". That would not prevent using these against unarmored people or objects, though (if the solution is of removable armor nature), but still that could be interesting. close combat weapons could use some "anti armor" tech that can't be adapted to ranged/gun weapon because of some reason (extreme sensitivity to shocks/explosions, for example, which would also make for interesting parameter to manage related to close combat weapons).
This reason could be adapted to a magical context (demon pacts guarding you against bullets, just like in Elric. With the twist that can't be done against regular weapons because of the functionning of the magic). There would be a balance between users of both sides but that would make it usually impractical, except for hunt maybe?

$\begingroup$I'm getting hints of some answers here, but not necessarily enough to qualify as a specific answer to the question. You might consider creating a bullet point list. It would require a pretty extensive edit though.$\endgroup$
– Paul TIKI -Monica come HomeJul 31 '17 at 20:13

$\begingroup$Ok, edited for expanding content and reflexion. I did not criticized the author question, but I don't see how it did not bring some answers, even partial. I added bullet list to clarify the reading.$\endgroup$
– Ando JuraiAug 1 '17 at 9:58

$\begingroup$I don't see how your physics based idea solves anything. I'd you do it like this, you would make any kind of projectile impossible.$\endgroup$
– Doomed MindAug 3 '17 at 7:22

$\begingroup$Changing inertia could affect all projectiles; but other solutions would not: gravity changes and hence increased perceived law of reaction has far higher effects with guns than with bow or crossbows or darts. Increasing friction exponentially would not limit the slowest projectiles (and would be close to Dune's shields).$\endgroup$
– Ando JuraiAug 3 '17 at 16:33

Eliminating all guns might be impossible, unless everyone who knew how to make black powder also died. However, reducing the world to muskets... maybe that's simpler. Modern weapons depend on remarkably precise metallurgy and manufacturing. No computers, no guns. No electricity, no computers.

Check out this timeline of gun development to see how far back you must push technology to have or remove guns from the mix. Basically pre-1300's if you want them completely gone. But that's guns.

Canons go back further, to at least the 1100's. Gun powder will pre-date canons. So you might be looking at having to reduce the world's technology to around year 1,000 A.D.

Regrettably, basic gun technology isn't that complicated. If an Encyclopedia Britannica is laying around, you'd have guns in your communities within a year.

$\begingroup$Most modern guns are only incremental improvements on designs that have been around since the world wars. John Browning didn't need a computer to design the 1911, for example.$\endgroup$
– DeolaterJul 31 '17 at 12:57

$\begingroup$Agree - if the metalworking knowledge is lost, then the same gunpowder used will be a greater danger to the gunner than the enemy. Why bother taking the risk of death for something that's pretty inaccurate anyway. All sensible people will get a crossbow instead.$\endgroup$
– gbjbaanbAug 1 '17 at 23:32

Guns are too easy to make in a culture that remembers the basics of their construction, the chemistry behind modern firearms' propellants etc... and the engineering needed to create cartridges etc... aren't though. I'd suggest that as ammo either runs out or goes off with age there will be an inability to keep modern firearms in use. That coupled to the dangers inherent in older models, especially old school black powder, which would cause people to lose fingers and faces when using makeshifts means that firearms just stop being viable for a time. Later as engineering capabilities, both physical and chemical, come back reliable guns are doable again but by then people have gone off the idea and don't pursue it.

S.M. Stirling has done some interesting work at both ends of this equation; the Nantucket Series, starting with Island in the Sea of Time, looks at what it takes to maintain a given level of technology when cut off from modern supply lines, and the Emberverse books, starting with Dies the Fire, looks at how a world without guns, and without any way to make guns work, evolves in the decades after the failure of energy dense technologies.

I don't think you can completely eliminate firearms given enough time, but you can delay their "reinvention" for a very long time. Firearms require a projectile, propellant, and a tube capable of containing the explosion of the propellant. The machine is just too simple. If you can make the production of one of the elements economically prohibitive, however, folks aren't going to look for a solution like firearms for a long time. In the interest of keeping the physics less twisted, lets make the most common propellant hard to make.

Have your apocalypse do 2 things:

1) Make it volcanic in nature. Enough so that elemental sulfur becomes very rare. Have all volcanoes that exude sulfur become dangerously active. It's hard to make gunpowder without it. Even though it's still out there, the incentive to harvest it is going to be greatly reduced. That will push back explosive developments for a while.

2) Have it kill off all birds. Entire wars were fought over islands that were covered in guano. The guano was a source of "saltpeter" and prized for it's usefulness in the production of black powder. There are other ways to get saltpeter like harvesting your own urine, but this is a slow and uncertain way.

This isn't going to stop the development of firearms, but it will make it not economical to develop them and it will slow their wide adoption down. Their is a reason why Black powder was so widely in use for so long. It was relatively cheap and easy to make and the black powder formula was pretty reliable. So make black powder expensive to produce.

Many answers have pointed out that there are many alternatives to Black powder. This is quite true. But there is a reason it took a while for "smokeless powder" to catch on. The metallurgy had to keep up. Put smokeless powder in a musket designed to use black powder, and you run a very real risk of having the thing blow up on you.

So you can settle for a scenario where the Bow and Arrow, atl-atl, or sling is just much more economical.

Because anyone who tries to make a firearm dies in a mysterious explosion.

Lateral reason: The post-singularity AI that caused/was-caused-by the Apocalypse has scattered spy nanobots everywhere. Rods from God are in orbit and the AI arranges to launch new satellites every so often to replace depleted launch platforms. Any time someone gets close to making a new firearm -- pop -- 11 tons TNT equivalent occurs at the site of potential innovation. (And anyone "nearby" who survives describes a truly astounding peal of thunder to go with that "lightning".)

Having described an AI with this much meatspace agency, there's no reason to assume it can't eliminate firearms innovations in otherwise difficult to reach spots by other means.

Okay here's a rather strange answer that just popped into my mind before anything else. This answer assumes some sort of governing regime that rules the survivors of the catastrophic event that happened.

You could have some sort of artificial intelligence chip embedded into people's minds at birth. What that chip does is detect any patterns or ideas regarding manufacturing certain weapons or designing systems that can manufacture weapons themselves. And the moment it detects such a thing, it prevents (harms maybe?) the person in some way.

I know it's a rather wacky idea and that we are talking about a post apocalyptic world where such ridiculously advanced technology would not be present. But maybe it's a technology whose remains survived the "apocalypse".

$\begingroup$Perhaps then, the chip was implanted in all people before the apocalypse, and perhaps has a way to replicate itself into offspring. The apocalypse actually never happened, the experience people feel is actually a simulation and as such can have non-linear laws. Thus, you could fabricate black power, and perhaps even do sufficient tests on it to theorise a firearm, but mysteriously, when you try to make it work it fails in some way that makes it infeasible as a solution. Careful though, you're making a world where the simulation plays 'whack a mole' with the actions of the people in it.$\endgroup$
– Ralph BoltonJul 31 '17 at 10:55

I don't think that you can make them "uncreatable" and it doesn't make any sense to try warping the whole environment around this sole concept.
Instead, I would make them useless, so that it doesn't make any sense to put efforts into creating such devices. In order to achieve that however, a more advanced forms or means of defense/offence must be introduced (that is of course, if you're not creating a Heaven there).

Not sure about the beings that are roaming this world, but they might have some psychic powers or something, that could just protect them from offenders. This in terms would make crimes avoidable and possibly predictable and if you don't introduce crimes - I don't even know what's going to be happening in this world :-) Don't get me wrong - I'm all pacifist and sh*t, but "breaking the law" is one of the most exploited concepts in general.

Another thing is to introduce air drag into the atmosphere (making it out of a substance, much like Newtonian fluid, but in a gas form), so that speedy projectiles could be slowed or even stopped at the moment of take off so much, that they can't cause harm (the question remains for close contact weapons //insert quote from the previous idea about more advanced weaponry//). As a side effect, no one could be killed after jumping from a skyscraper, no speeding (no collisions), but also - no speedy means of transport, which would require something like the Hyper-loop to be built, which of course would increase the murder rate within this contraption :)

It depends on how far back you want their technology to go, but Horizon: Zero Dawn offers a very good example of how this can be done. In order to have technology regress by a considerable margin, you have to destroy all current human knowledge, and I mean everything, including any and all knowledge repositories and backups. Fry every piece of electrical equipment on the planet, remove all humans from the equation (possibly by killing all or most of them), add enough time (At least 1,000 years) for anything physical to have completely eroded, and then reintroduce humans to the equation. In this example, they will have regressed to the point of tribal collectives, hunting with arrows and spears.

You can adjust the amount of devastation caused by your cataclysm, as well as how far it spreads, as necessary to destroy / preserve as much technology and knowledge as you want.

Like "Revolution", you could have nanobots that permeated everything and have a destructive effect on explosive compounds, they eat them or otherwise break them down so they won't work for long after manufacture. This does not stop compressed air guns from being built.
https://en.wikipedia.org/wiki/Revolution_(TV_series)

Like Frank Herberts' "Dune" you could have a technological reason why non-explosive-projectile weapons are nearly pointless. The small wearable energy shields that people wore repulsed fast-moving objects like bullets. But that isn't likely in a post-apocalyptic setting.

$\begingroup$What about bows :) or spears, they don't explode, but could kill ..$\endgroup$
– g00dyJul 31 '17 at 11:08

$\begingroup$Right. My answer doesn't cover all the possible cases. It may be useful anyway I hope$\endgroup$
– Fez VrastaJul 31 '17 at 11:25

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$\begingroup$I didn't mean it in that way :) more like for you to elaborate - like if one shoots an arrow or throws a spear - the air friction would cause an ignition with the same effect as shooting a gun for instance :-D$\endgroup$
– g00dyJul 31 '17 at 11:51

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$\begingroup$@g00dy that would be pretty dangerous... You should move slowly to avoid any friction lol$\endgroup$
– Fez VrastaJul 31 '17 at 11:52

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$\begingroup$Of course this brings up the issue of fire in general. People would have to learn to start fires remotely to burn off the volatile gases within an area, so they could run a furnace/etc to do any other manufacturing, otherwise they won't even be able to progress beyond the stone age. It could be that the gas is just sparse enough that there's a chance of ignition (enough to deter the idea of using firearms), but if it does ignite only creates a local explosion, otherwise the world would get swept in firestorms constantly.$\endgroup$
– Doktor JJul 31 '17 at 17:38

$\begingroup$Flammable air will burn during next thunderstorm. This solution is very, very short term. Also, it would prevent electric and petrol engines from working, people from making campfires et cetera. Also, atmosphere as acidic as you say would also damage eyes of anyone there, rendering the setting to be unsuitable for most kind of stories... same for low visibility.$\endgroup$
– MołotJul 31 '17 at 12:19

$\begingroup$Well in a shelter setting with the outside like this, it makes some sense, actually...$\endgroup$
– Ando JuraiJul 31 '17 at 16:10

So what about magnetic fields? If there were strong, magnetic forces, they should render most bullets useless.
The downside is that other metallic objects would be useless too. And weapons like Bows would still work.

The cause of those magnetic forces could be a seal inside of the planet that has been broken, due to the same thing that caused the apocalypse in the first place or an alien force, that deploys technology wich is able to create those magnetic fields on the planets they conquer.

$\begingroup$What about using non magnetic metals as projectiles? A lot of them are not sensible to magnetism, you know?$\endgroup$
– Ando JuraiJul 31 '17 at 16:08

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$\begingroup$Most modern bullets are made out of lead, with a thin copper jacket over some or all of the projectile. Neither one is magnetic. And any material softer than the barrel, and strong enough to survive the burning gun powder and forces involved could be made to work passably well. As for the firearm itself, austenitic stainless steels could probably be used, which would drastically reduce the effects of magnetic fields.$\endgroup$
– Jay KominekJul 31 '17 at 19:32

A society in space without "shields" would have to develop some other weapons besides guns because as the first comment sais "A single errant shot" can end everyone. That is a culture likely not to use them. That being said, you cant possibly expect projectiles in general to cease to exist. All a gun is a projectile thrower. In general anything moving fast is dangerous to anything not moving at a relative speed to that object. That means in order for your civilization to not have projectile weapons you get to have nothing and you will never progress to anything past the medieval era. People could throw knives in the space ship cor-doors, and inevitable someone would use rubber hosing to catapult a knife or rock or whatever and that kind of cancels out the whole lack of guns thing. Not to mention your spaceship could be interpreted as a bullet.

The best case scenario for your society without guns that somehow has projectiles and does not use them for violence ever that is also sustainable realistically is a society on a planet or moon like environment. In this society one step out of line from anyone could result in the death of everyone. Everyone has to have a place and cooperate. There is no room for chaos at all. Additionally because it is the environment enforcing this culture and not a totalitarian government, this society is not necessarily oppressed. It would have a whole "us against the world" mentality, which could potentially bring out the best in people.
On a side note they would probably just use darts as enforcement tactics. You cant have the police engaging in fistfights and/or swordfights (Which are terrible ways to die/kill people and significantly more inhumane than guns), and you also cant just "expect" every single person ever born to conform to your societies rules forever. There will be a chaos factor no matter how reduced, and there needs to be ways to overcome it without undo effort.

Global warming causes the apocalypse or the apocalypse causes extreme global warming, this leads environmentalists to produce a plant designed to rapidly harvest CO2 from the air and convert it into O2.

BUT the plant spreads out of control like a weed and starts pumping out excess O2. Life can still survive because it loves O2 but that O2 is extremely explosive... which means that anyone firing a gun causes the surrounding air to explode encompassing themselves in fire as well.

Meaning people would stop firing guns because they'd know how bad of an idea it was. Would also make cars and some other commonly used things, possibly even anything electrical that could cause a spark, to be dangerous.

I prefer the first but 2nd scenario, similar idea:
Could potentially be some other chemical that is spread worldwide intended to destroy the enemies weapons supply by causing spontaneous explosions of gun powder that causes the explosion... which lingers and leaves people unable to fire guns.

John Ringo did a couple books that I can't remember the name of in which he applied the concept that traditional firearms no longer worked, then kicked off a major war/apocalypse. In short we had developed a master computer and nano tech to the point that it was capable of near instantaneous energy manipulation and teleportation. Somewhere along the line it was decided to program the computer to defuse and disperse any source of energy/pressure significant enough to fire a projectile or cause structural damage. To the point that it was commented that the computer was able to contain the super volcano under yellow stone when it started to erupt and most of the world didn't notice anything had happened.

When the apocalyptic trigger occurred and society lost the ability to instruct the computer, it continued on automatically enforcing the rules it had on file. So as the war spools up, the characters are trying to figure out ways to recreate old tech like fire arms and the internal combustion engine. The best they can come up with is low powered steam, and bows & arrows, because anything more the computer/nano tech defuses before they can get any use out of it.

Weaponized nanobots from last global war, designed to set off powder and explosives spread world-wide and are self-perpetuating, but no technology exists in post-apocalyptic world to neutralize them. You also could add nanobots preventing electricity generation and/or semiconductor electronics, if so required by your story. Granted, nanobots are a somewhat handwavium device, but, on the other hand, at this point we don't know what they are truly capable of.